Porphyria cutanea tarda (PCT) is the most common of the porphyrias and is due to a marked deficiency of uroporphyrinogen decarboxylase in liver. The uroporphyrinogen decarboxylase deficiency found in PCT is primarily acquired. The aims of this study are as follows. 1. Vitamin C status is assessed before and after standard tretment (either repeated phlebotomy or low-dose chloroquine) by measuring vitamin C levels in serum and white blood cells. This will test the hypothesis that vitamin C is an antioxidant in the liver and that its deficiency may contribute to activation of PCT. 2. Activity of CYP 1A2 is assessed in vivo using 13C- caffeine and 13C-methacetin breath tests and also by measuring the clearance rate of caffeine from blood and urinary excretion of caffeine metabolites. 3. The role of hepatitis C infection is assessed by (a) observing the frequency of recurrence of PCT by repeated phlebotomy or low-dose chloroquine on abnormalities in liver function tests and on the presence of hepatitis C antibody and hepatitis C RNA in plasma; and (b) by observing the frequency of recurrence of PCT in patients who are treated with interferon alpha-2b as compared to patients who are not treated with this drug. Standard serum chemistries as well as a [13C] phenylalanine breath test are used to assess liver function. The indications for treatment with interferon alpha-2b will be the standard indications for use of this drug in patients with hepatitis C and will not be inflenced or specified by this protocol. Patients are treated first for PCT, because treatment for PCT is almost always successful, whereas treatment for hepatitis C is usually not.